Surface analysis methods have advanced to achieve atomic resolution using a probing tip having an apex of atomic dimensions on surface probing devices. The probing tip is usually a tapered silicon structure, referred to as a stylus, with a base attached to a cantilever arm and a sharp apex that interacts with the surface being probed. The parts of a surface probing device include a stylus or aperture, a cantilever arm and a mounting section. In addition, a surface probing device may have an electrical connection from the stylus, through the cantilever arm, connected to external circuitry or may also have a reflective coating on the cantilever arm. The electrical connection and the reflective coating provide different ways to measure the response of the stylus apex to the surface being analyzed.
An apparatus that uses a surface probing device for surface analysis involves a scanning process. During the scanning process the stylus apex will respond to a surface characteristic. The response is monitored and generally held constant through a feed-back system that causes a slight change in the cantilever arm position. Two notable examples where these general principles apply are scanning tunneling microscopy (STM) and atomic force microscopy (AFM).
In STM a stylus apex of atomic dimensions on a cantilever arm follows the contour of a sample surface. Electrons tunnel through a near-field vacuum between the conductive apex of the stylus and a conducting sample creating a tunneling current. The tunneling current is very sensitive to changes in the distance between the stylus apex and the conductive sample surface. A feedback system is used to monitor and control the tunneling current at a constant value, and optical detection techniques such as interferometry or laser beam deflection are used to measure the resultant cantilever arm deflection during scanning.
AFM uses a stylus that is mounted on a cantilever arm that has a small spring constant and scans a surface such that repulsive inter-atomic forces between the surface and the stylus apex cause deflections in the cantilever arm position. Again, a feedback system is used to monitor and control the forces between the tip and sample, and an optical detection technique such as interferometry or laser beam deflection are used to measure the resultant cantilever arm deflection during the scanning process.
A new apparatus that uses a silicon stylus mounted on a cantilever arm is a scanning probe potentiometer (SPP). The SPP apparatus adapts the principles of light-addressable potentiometric sensor (LAPS) and combines them with a micro-scanning structure similar to that used in AFM and STM. The LAPS apparatus was developed to potentiometrically measure changes in pH, redox potential and transmembrane potential in a highly sensitive manner (see Hafeman et al., Science, Vol. 240, 1182 (1988)). The LAPS consists of an insulated semiconductor device which is immersed in an electrolyte. The sample of interest is placed on the surface of the insulator (at the insulator-electrolyte interface) and a bias voltage is generated such that the solution is negative with respect to the semiconductor. The insulator consists of a pH-sensitive material, such as silicon nitride or silicon oxinitride, or other charge-sensitive material. Electron-hole pairs are created in the semiconductor by a pulsed intensity-modulated light source, resulting in separation of charges in an area called the depletion region of the semiconductor. The electrons in the depletion region migrate, thereby causing a current of a magnitude that depends on the bias voltage and the chemistry of the sample at the adjacent insulator-solution interface. This movement of electrons yields a detectable alternating current in the external circuit. The adaptation of stylus apex surface probing device to LAPS provided a method for mapping charge distributions accurately for small surfaces.
Another application that uses a stylus supported by a cantilever arm construction is near field optical scanning microscopy (NSOM). Quate et al. describes NSOM in U.S. Pat. No. 5,354,985. In this technique the stylus and cantilever arm are a wave guide, whereby light escapes through a small hole or aperture in the apex of the stylus and is directed towards the sample. The cantilever arm is vibrated and variations in the resonant frequency are detected.
Several methods for fabricating surface probing devices with a stylus and cantilever arm have been reported. Bothra et al., U.S. Pat. No. 5,540,958, describe a method for making a stylus on a cantilever arm by first etching a silicon wafer with a mask to produce protruding shapes of a predetermined size and then depositing a second layer such as silicon oxide, by electron cyclotron resonance. Shimada et al., U.S. Pat. No. 5,546,375 describe making a stylus by forming a recessed cavity in a silicon wafer. The cavity is then used to define the structure of the stylus. In U.S. Pat. No. 5,399,232, Albrecht et al. describe a method of fabricating a cantilever arm and stylus again by forming a depressed area in a silicon wafer and using the depressed area to define stylus shape. In U.S. Pat. No. 5,581,083, Majumdar et al. describe a method for producing a hole at the apex of a stylus. The method uses an applied voltage to a metal coated tip causing evaporation of a metal coating and exposing the underlying silicon apex.
The combination of a stylus and cantilever arm is important for many modern surface probing methods. Each method of analysis requires a stylus and cantilever arm with properties tailored to the application at hand. Prior art teaches methods to fabricate silicon styluses on cantilever arms where the cantilever arms are made from silicon. One difficulty that can arise in fabricating surface probing devices with silicon cantilever arms is that the thickness of silicon is difficult to control by an etching processes. Additionally, it is beneficial for applications such as SPP to make surface probing devices that contain an electrically isolated silicon stylus that is connected to external circuitry through a conductive metal deposited on the nitride cantilever arm. Therefore, it is important to have methods for fabricating styluses and cantilever arms for surface probing devices, whereby the thicknesses of the cantilever arms are easy to control during the fabrication process and the cantilever contains an electrically isolated silicon tip.